This invention relates generally to field effect transistors, and more particularly the invention relates to insulated field effect transistors (IGFETS) and metal oxide silicon transistors (MOS) having a floating gate for use in electrically erasable, programmable read-only memory (EEPROM) cells as Flash EEPROM cells.
The MOS transistor has source and drain regions separated by a channel region with the conduction of the channel region controlled by voltage biasing an overlying gate. The Flash EEPROM device has the structure of a metal-oxide-semiconductor field effect transistor (MOSFET) but additionally includes an electrically isolated gate, or floating gate, to store charge. Controlling the amount of charge on the floating gate alters the threshold voltage and creates a nonvolatile memory function. The cell is read in the same manner as a MOSFET. Flash memory development is therefore driven by some of the same concerns as MOS technology, but to the demands for scalable devices with high access times and low leakage currents, it adds the requirement of efficient, controllable gate currents, and channel hot electron (CHE) programming used in conventional flash cells. As the drain and gate voltages are lowered, however, this method becomes less efficient.
Since flash memory is used mostly in mobile applications, low power operation is desirable. For programming with lower drain and gate voltages, a negative substrate voltage may be applied that produces a different programming mechanism that is more effective than CHE programming at these lower voltages. Channel initiated secondary electron (CHISEL) injection occurs when a substrate bias is applied, and the mechanism is also known as substrate current induced hot electron (SCHE) injection. Channel electrons gain energy as they travel to the drain and produce primary impact ionization. The holes generated in the drain then travel back across the drain junction and gain energy as they pass through the depletion region where they can produce secondary impact ionization (SII). the secondary electrons created by SII can travel back to the Si-SiO.sub.2 interface and be injected into the floating gate.
The present invention is directed to a floating gate field effect transistor in which secondary electron injection is enhanced when programming the floating gate.